The present invention relates to a rubber composition or, more particularly, to a rubber composition compounded with a finely divided reinforcing filler capable of exhibiting greatly improved flowability and workability in the roll milling and molding works and capable of giving a vulcanizate having excellent mechanical properties, heat resistance and other properties.
Needless to say, carbon black is the most widely used filler, among various kinds of reinforcing and non-reinforcing fillers, in rubber compositions based on natural or synthetic rubbers. This is because particles of carbon black have an oleophilic or organophilic surface having high affinity with polymer molecules of the rubber along with the extremely large specific surface area thereof so as to exhibit an outstandingly high reinforcing effect. A problem in carbon blacks as a filler in rubber compositions is that the rubber compositions compounded with a substantial amount of carbon black are always black in color so that carbon black cannot be used when a rubber article of white or a light color as well as when an aesthetically desirable color of the rubber article is desired.
As a class of light-colored fillers capable of giving a rubber article of a color other than black, finely divided silica fillers can be used advantageously in a rubber composition and a rubber composition compounded with a silica filler gives a rubber vulcanizate having excellent heat and aging resistance, tear strength and resistance against crack formation by bending as well as good adhesion to the substrate surface on which the composition is vulcanized.
On the reverse side of the above mentioned advantages, silica fillers have a problem in respect of the affinity with and the dispersibility in the matrix of the polymeric molecules of natural and synthetic rubbers because a large number of silanolic hydroxy groups are usually bonded to the silicon atoms on the surface of the silica filler particles. Therefore, a rubber composition compounded with a silica filler sometimes has an unduly increased consistency to greatly decrease the workability of the composition and the properties of the vulcanizate obtained from the composition.
Various attempts and proposals have been made hitherto to solve the above mentioned problem of the poor dispersibility of silica fillers in a rubbery polymer. For example, it is proposed that a silica filler-loaded rubber composition is compounded with an alcoholic compound such as polyethylene glycols and the like or a surface active agent though with only a limited effect on the dispersibility of the filler as compared with carbon blacks in general. It should also be noted that polyethylene glycols are contained in the composition as a mere additive ingredient which never pertains to the crosslinking reaction so that addition of such an additive sometimes adversely affects the properties or, in particular, permanent compression set of the rubber vulcanizate prepared from the rubber composition. It is also proposed that finely divided silica fillers are subjected, prior to compounding with a rubber, to a surface treatment with an alcoholic compound, organosilane or organopolysiloxane compound and the like. This method is indeed effective in improving the dispersibility of the filler in the rubber matrix and could be widely practiced if it were not for the large costs required for the pretreatment of the filler. No sufficient improvement can be obtained in the dispersibility of the filler by compounding a rubber composition simultaneously with a silica filler and an organosilane compound.
On the other hand, Japanese Patent Kokai 61-225243 teaches that acrylic rubbers can be imparted with improved long-term heat resistance and resistance against greases and water by compounding with a non-carbon filler but nothing is mentioned there on the problem of the dispersibility of the filler in the rubbery matrix and the workability of the rubber composition. Further, Japanese Patent Kokai 55-31817 teaches that an improving effect can be obtained in the permanent compression set of a vulcanizate of a silica filler-loaded natural or synthetic rubber composition by admixing with an organopolysiloxane. Admixture of an organopolysiloxane alone, however, has little effects on the dispersibility of the silica filler. In particular, acrylic rubbers and the like having a relatively low green strength give a composition having a high Mooney viscosity when compounded with a silica filler due to the particularly poor dispersibility of the silica filler therein so that a rubber vulcanizate having a complicate configuration can hardly be prepared from such a rubber composition. This difficulty is due to the abnormal increase in the torque of the composition in the step of temperature elevation in the molding process of the composition to greatly decrease the flowability thereof without decrease the torque at the intitial stage of the temperature elevation as in most of other types of rubber compositions.
As a class of synthetic rubbers, silicone rubbers are compounded usually with a finely divided silica filler as a reinforcing agent. Vulcanizates of such a silica filler-loaded silicone rubber have excellent heat and cold resistance and weatherability as well as good electrical properties so that silicone rubebrs are widely used in various fields of industries. One of the disadvantageous properties in silicone rubbers is that vulcanizates of silicone rubbers usually have relatively poor resistance against oils and solvents, e.g., gasoline, exhibiting large swelling therewith even after complete crosslinking and curing by use of an organic peroxide and the like as a crosslinking agent. This defective property is an inherency of the dimethylpolysiloxane which is, in most cases, the principal ingredient in silicone rubber compositions.
With an object to solve the above mentioned problem of poor oil resistance in general purpose-grade silicone rubbers, an oil resistant-grade silicone rubber has been developed by replacing a part of the methyl groups in the dimethylpolysiloxane with 3,3,3-trifluoropropyl groups. Such special grade silicone rubbers, however, are not widely employed in practical applications due to the extremely high costs for the preparation of the fluorine-containing organopolysiloxane.
In connection with the problem of oil and solvent resistance of a rubber vulcanizate, acrylic rubbers are known to be excellent in this regard along with fairly high heat resistance so that they are highlighted in the applications to automobile parts. Acrylic rubbers, however, have other problems in the relatively low cold resistance and poor workability in roll milling and molding so that it is eagerly desired in the rubber industry to improve acrylic rubbers in these regards and various proposals and attempts have been made.
For example, it has been proposed to compositely combining a silicone rubber and an acrylic rubber to compensate the defective properties of one rubber with the excellent properties of the other. Thus, Japanese Patent Kokai 55-7814 and 60-152552 teach preparation of a silicone-acrylic rubber blend having good workability in roll milling and molding without the problem in the compatibility of different polymers and capable of being cured by using an organic peroxide by compounding a copolymer of an organopolysiloxane having ethylenically unsaturated groups, e.g., vinyl groups, and an acrylic acid ester with a rubber blend of an unvulcanized organopolysiloxane gum and an unvulcanized acrylic rubber.
The above described composite rubber composition also have several disadvantages. For example, the additive copolymer of an organopolysiloxane having ethylenically unsaturated groups and an acrylic acid ester may sometimes be gelled when the molecular weight thereof is high as a consequence of the polymerization reaction between the vinyl groups and the acrylic unsaturated groups. Further, the additive copolymer is not co-crosslinkable with the rubbery polymers in the crosslinking reaction with an organic peroxide and the like because the copolymer lacks crosslinking points such as unsaturated groups so that the vulcanizates obtained from the composite rubber composition may sometimes have relatively poor mechanical properties, especially, when the added amount of the additive copolymer is increased with an object to fully obtain the improving effect by the addition thereof.
While it is desirable in the above described composite rubber composition that the amount of the organopolysiloxane relative to the acrylic rubber should be as small as possible in respect of the costs and the oil resistance of the vulcanizate, the amount of the organopolysiloxane must sometimes be increased to exceed 50% by weight in order to achieve substantial improvement in certain properties such as cold resistance of the rubber vulcanizate as is evidenced by the results of the TR test according to ASTM D-1329 or Gehman torsion test according to ASTM D-1053 which give a measure of the flexibility of a rubber vulcanizate at low temperatures.
Alternatively, Japanese Patent Publications 54-3512, 54-6271 and 55-14086 disclose a copolymer based on an acrylic polymer and capable of giving a cured product having improved heat and cold resistance, weatherability and impact strength prepared by the copolymerization of an acrylic monomer and an organopolysiloxane. These copolymeric polymers, however, are readily gelled when a polymer of a high degree of polymerization is prepared because the copolymerization reaction is performed in most cases by utilizing the radical-polymerizability of the vinyl-containing organopolysiloxane so that the copolymer can rarely give a rubber vulcanizate having high mechanical properties but the application thereof is limited to coating compositions and resinous products.
Japanese Patent Publication 54-6271 also discloses copolymerization of a mercapto group-containing organopolysiloxane and a vinyl monomer including acrylic monomers. The copolymer disclosed there, however, cannot be used as an ingredient of a rubber composition because the organopolysiloxane used there contains trifunctional RSiO.sub.1.5 units and tetrafunctional SiO.sub.2 units in addition to the lack of crosslinkable monomeric moiety derived from an active halogen-containing monomer, epoxy group-containing monomer or .tbd.Si--CH.dbd.CH.sub.2 group-containing monomer.
As is taught in U.S. Pat. No. 3,622,547, Japanese Patent Publication 49-13215 and Japanese Patent Kokai 58-222129, known crosslinking agents for an acrylic rubber, of which active halogen-containing groups provide the crosslinking points, include trithiol-S-triazine compounds, sulfur and metal soaps. Although di- or trithiol-S-triazine compounds as a vulcanizing agent of an acrylic rubber serve to considerably improve the heat resistance of the rubber vulcanizate, they are not widely used in practical applications because of the relatively poor storage stability of the rubber composition compounded therewith. The above named patent documents, however, are silent on the problem of the poor dispersibility of a finely divided reinforcing filler in the rubber composition.